In factories that manufacture semiconductor substrates, liquid crystal display panels or the like, it is necessary to keep plate-like workpieces (semiconductor wafers or glass plates) used as raw materials, as planar as possible and to avoid damage to the workpieces. It is also necessary to enable the workpieces to be stacked and to minimize a stacking height in order to save a handling space. To meet these requirements, proposals have been made of trays that contain these plate-like workpieces (hereinafter also simply referred to as “workpieces”).
Further, the Unexamined Japanese Patent Application Publication (Tokkai) No. 2005-191419 (FIGS. 1 to 4) proposes a storage container that accommodates stacked trays. The storage container (hereinafter referred to as the “pod”) does not fully utilize this advantage of the trays of allowing stacking height to be reduced to a fraction of that of conventional cassettes that accommodate workpieces on stages.
On the other hand, in connection with conveyance of trays, that is, workpieces, the above factories have been requesting development of a system that can deal with multiproduct small lots to improve conveyance efficiency, that is, a system that can perform efficient conveyance by allowing its conveyance form to be changed as required.